Methane and nitrogen cycling within the tropical tree canopies in the Peruvian Amazon wetlands

Tropical forests occupy a substantial share of the Earth’s forested land area. In addition to serving as major carbon reservoirs, these ecosystems influence the global greenhouse gas (GHG) balance by acting both as sinks and sources of methane (CH₄) and nitrous oxide (N₂O). Despite their importance, and sensitivity to climate change, the biogeochemical functioning of tropical forests remains insufficiently understood, particularly with respect to processes occurring in above-ground vegetation.

In this study, we investigated GHG cycling in tree canopies at two peat-swamp forest sites in the Peruvian Amazon: a protected palm swamp reserve Quistococha (3.83417° S, 73.31889° W) and a nearby secondary peatland forest Zungarococha, which served as a reference system.

Field campaigns conducted in November 2023 and May 2024 quantified potential CH₄ and N₂O production and uptake in leaves and twigs of three to four representative tree species (Symphonia globulifera, Mauritia flexuosa, Hevea sp., Tabebuia sp.). Aerobic incubations were performed on-site over 48 hours, with daily gas sampling for analysis via gas chromatography. Biological nitrogen fixation was assessed using ¹⁵N isotope labeling over a 72-hour incubation. In parallel, branch material was collected for metagenomic characterization of epiphytic and endophytic microbial communities.

Across all tree species, leaves exhibited small but statistically significant fluxes of both CH₄ and N₂O. In contrast, twig samples displayed species-specific behavior: Hevea sp. acted as a weak sink for both gases, whereas Symphonia globulifera was a consisted source. Considerable variability was observed not only among species but also between the two forest sites within the same species. Nitrogen fixation activity was detected in three of the four studied taxa. Metagenomic analyses revealed the genetic capacity for complete denitrification pathways and for N₂ fixation, while genes associated with nitrification (amoA) were rare. All analyzed tree species contained a high diversity of methanotrophic bacteria. Reads related to methanogenic archaea suggested presence of variable CH₄ production pathways.

Our findings highlight tropical tree canopies as active components in the GHG cycling. By linking gas fluxes with the microbial functional potential, this work provides new insights into how above-ground plant–microbe interactions can shape ecosystem-level GHG balance in tropical peatland forests.